The present invention relates to systems and methods for recovering metal from salt cake and similar compositions and, more particularly, relates to such systems and methods employing staged eddy current separators to recover product having significantly high metal concentrations from salt cake or similar matter.
Salt cake is obtained in the remelting of metal scrap, such as aluminum scrap or dross. In the remelting of aluminum or aluminum bearing materials, a flux is employed that includes a mixture Of salt, primarily potassium chloride and sodium chloride, and a fluoride compound typically cryolite. The flux is employed to remove impurities from the remelted scrap, to reduce oxidation of the metal and to enhance separation of the metal from non-metal constituents. When the remelting furnace is tapped after the remelting process, pure (i.e., refined) metal is obtained as a product. A byproduct of the process that is also obtained is salt cake.
The salt cake includes the flux, impurities that were contained in the metal, metal oxides, and remaining metal remnants not separated as pure metal product through the remelting process. The metal remnants remain in the salt cake because they do not coalesce into larger metal pieces during the remelting process. The larger metal pieces are retrieved as product, but the remnants are not separated from the salt cake. In cases in which aluminum is the recovered metal, the salt cake, including aluminum remnants trapped in the salt cake, is typically disposed as waste, for example, by dumping in a landfill.
Several techniques for separating aluminum from salt cake are conventional. One of these techniques employs crushing and screening to separate aluminum from non-aluminum particles of salt cake. Several stages of crushing and screening have at times been employed. In the case of several stages, the screening at each successive stage removes smaller particles than those removed in the prior stage. A significant disadvantage of the technique is that much of the aluminum in the salt cake is not retrieved and is, instead, dumped in landfills with the salt cake.
In another technique, the raw material is crushed and then ground to smaller particles. The grinding serves to flatten aluminum particles of the raw material, making them larger in two dimensions than the non-aluminum particles of the raw material. The particles are then screened in several stages of different size screens to remove the particles of the raw material according to size. The larger particles removed have a higher concentration of aluminum. A significant disadvantage of the technique, however, is that the process is expensive. In addition, the resultant non-metallic material is so fine as to be dusty and otherwise difficult to handle in a landfill.
Another conventional technique for removing aluminum from salt cake dissolves the salt of the cake, thereby freeing the aluminum and other non-dissolvable particles. In this technique, the salt cake is first crushed and ground. Water is then added to dissolve the salt. The solution is wet screened to recover the aluminum and other non-dissolvable particles. Disadvantages of the technique, however, include wetting of the salt cake that causes oxidation of some of the remaining aluminum, a wet, non-metallic byproduct which must be removed by filtering, and salt solution which must be discarded or from which salts must be recovered, for example, by an additional process such as evaporative crystallization. In addition, the process involves significant energy consumption and operating and capital costs are high.
Therefore what is needed is a method for aluminum recovery from salt cake and a system for performing that method that overcome the disadvantages and problems of the foregoing techniques and systems.